Brassica seeds, including rapeseed, canola and mustard seeds, are a potential source of high quality protein suitable for human consumption. The defatted meals that can be obtained from these seeds contain about 40% w/w protein with a well-balanced amino acid composition, and have excellent functional properties. However, the use of Brassica seeds as a protein source is limited by the presence of certain undesirable toxic and anti-nutritional components, including glucosinolates, phytates, and phenolic compounds. The concentration of these undesirable components must be substantially reduced before these types of protein isolates are suitable for human consumption.
Glucosinolates are hydrolyzed in enzymatic reactions to form compounds that can interfere with thyroid function and cause liver and kidney damage at high concentrations. Phytates are strong chelating agents that bind to polyvalent metal ions in the body including iron, calcium and magnesium, rendering them unavailable for metabolism. Phenolic compounds impart an unpleasant bitter taste and a dark colour to the final protein products.
Phenolic compounds are particularly difficult to remove because some of the phenolics bind to the proteins in an aqueous media to form relatively large phenolic-protein complexes. Xu and Diosady (Food Res. Intl. 33:725 2000) characterized the canola protein-phenolic interactions in an aqueous media, using a series of chemical treatments followed by membrane separations. The results suggested that approximately 50% of the total extracted phenolic compounds formed complexes with canola proteins through ionic bonding (˜30%), hydrophobic interactions (<10%), hydrogen bonding (<10%), and covalent bonding (<10%). Although these figures may seem minor, if not removed, they could be concentrated to high phenolic compound levels in the protein isolates which represent only a small fraction of the meal mass.
In U.S. Pat. No. 4,889,921, Diosady et al. discloses a process for the production of protein isolates from rapeseed, including the steps of alkaline extraction and isoelectric precipitation to obtain a precipitate from which a first product stream of protein is recovered. The depleted solution from the precipitation stage is subjected to ultrafiltration followed by diafiltration and drying to obtain a second product stream of recovered protein. These two protein isolates were produced with a combined protein recovery of over 70% of the protein present in the seed. Both products were of high protein content (>90%), essentially free of glucosinolates (<2 mol/g), low in phytates (<1%), and had desirable functional properties for a variety of food applications. However, both of the protein isolates had an unpleasant bitter taste and a dark colour. These unacceptable organoleptic properties were attributable to the phenolic compounds that were left behind in the protein isolates.
In the U.S. Pat. No. 4,889,921 patent, membrane processes are used to concentrate and purify the protein isolates. These processes separate dissolved components on the basis of their molecular sizes. Specifically, the membranes reject and retain large molecules in the retentate, while allowing small molecules (impurities) to pass through into the permeate. These processes are effective at removing the glucosinolates and the phytates, as they are relatively small and pass through the pores of the membrane. However, the relatively large phenolic-protein complexes tend to be rejected by the membrane, and thus remain behind in the retentate along with the protein isolates. Further, the precipitate from the precipitation stage also includes phenolics and bound phenolics.
There still exists an ongoing need for a method for producing protein isolates derived from Brassica oil seeds that have low concentrations of glucosinolates, phytates and phenolic compounds.